(CNN) -- Investigators looking for the source of a salmonella outbreak linked to tomatoes will focus on farms in Mexico and Florida, federal health authorities said Friday.

Since April, more than 500 people have contracted the same strain of salmonella, linked to raw tomatoes.

The tracebacks "have taken us from point of consumption all the way back to certain farms in Mexico and Florida," said Dr. David Acheson, associate commissioner for foods at the Food and Drug Administration.

The agency will send teams of investigators to farms in both locations this weekend as well as to the pathways from those farms in an attempt to determine where the contamination occurred, he said.

The tomatoes may not have been contaminated on a farm, he stressed; the contamination could have occurred in a packing shed, warehouse, supplier chain or distribution center.

"We are going to all of those places to see if there are any problems that could indicate how or why these tomatoes got contaminated," he said.

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The reported advance in the investigation came as the toll mounted, with 552 people identified as having contracted the strain of Salmonella Saintpaul since April in 32 states and the District of Columbia. It is one of the biggest outbreaks of tomato-caused illness in history, officials said. See where the cases have been reported »

Though the number of reported victims has risen dramatically in recent days, that does not signify a large number of new infections, Acheson said.

Instead, he credited improved surveillance and laboratory identification of previously submitted strains for the increased number.